1. Field of the Invention
The present invention generally relates to an image forming apparatus such as a digital copier, a printer, a facsimile and so on of an electric photographic type, provided with a writing device that performs image exposure on a photoconductive image carrier by deflecting a light beam emitted from a light source by a deflector.
2. Description of the Related Art
A brief description of the schematic configuration of such an image forming apparatus is given below by taking a digital copier as an example. The digital copier of an electric photographic type includes an automatic document feeder that feeds document sheets one by one from a pile of document sheets onto a contact glass and sets the document sheet thereon. The digital copier further includes an image reading unit that optically scans the document sheet on the contact glass and forms an image on a CCD sensor, and an optical scan unit that forms an (electrostatic latent) image on a photoconductive image carrier by deflecting a light beam emitted from a light-emitting semiconductor based on a digital image signal. The digital image signal is converted from an analog image signal that is obtained by photoelectrically transforming a reflective light image of the document sheet formed on the CCD sensor by an analog-to-digital converter. The digital copier further includes a transfer unit that transfers an image developed from the electrostatic latent image formed on the photoconductive image carrier onto a transfer paper and a feeder unit that feeds transfer papers to the transfer unit.
Among these components, the optical scan unit that configures a writing device may be exemplified by an optical scanning system shown in FIG. 1. A laser beam emitted from a semiconductor laser (LD) 1a, which is provided in the interior of a semiconductor laser unit (LD unit) 1 as a light source, become a substantially parallel-light beam by passing through a collimator lens 1b and become a chief ray L having a cross-section shaped as necessary for image formation by an aperture 2. The chief ray is condensed in the sub-scanning direction by a cylindrical lens 3 and is reflected by a mirror 4 so as to enter a polygon mirror 5, which serves as a deflector. The chief ray L entering the polygon mirror 5 is deflected by the reflective side surface of the polygon mirror 5 that rotates with a high velocity in the direction indicated by the arrow, and the deflected chief ray L enters an Fxcex8 lens 6. By entering the Fxcex8 lens 6, the constant angular velocity scanning with the chief ray is transformed into constant linear velocity scanning with the same. The chief ray exits to the exterior of the writing device and is focused on a photoconductor drum 7, which includes the surface to be scanned and thus an electrostatic latent image is formed thereon. In FIG. 1, the numerical references 9a and 9b represent a synchronism detection mirror provided outside of the image region and a synchronism detection sensor that detects a reflection beam from the synchronism detection mirror, respectively. A synchronization signal as a reference for the beginning of scanning in the main scanning direction can be obtained from the output of the synchronism sensor 9b. In addition, the Fxcex8 lens 6 has an optical face tangle correction function.
FIG. 2 and FIG. 3 show a configuration of one example of the writing device 10 provided with the above-mentioned optical scanning system. FIG. 2 is a top plan view of such a configuration and FIG. 3 is a sectional side view of such a configuration. As is apparent from FIG. 2 and FIG. 3, in the actual writing device 10, the polygon mirror 5 is rotationally driven by a polygon motor 8. A soundproof glass 5a is fastened to a polygon cover 8a of the polygon motor 8. The light beam entering the polygon mirror 5 from the mirror 4 and the light beam exiting the polygon mirror 5 to the Fxcex8 lens 6 pass through the soundproof glass 5a. The rotational velocity of the polygon mirror 5 is determined based on the rotational velocity of the photoconductor drum 7, the writing density of the writing device 10, and the number of side surfaces on the polygon mirror 5.
However, in such a writing device of the image forming apparatus according to the related art, some undesired light beam may occur from the LD unit 1, which light beam is different from the chief ray L and does not contribute to the proper image formation, due to the side lobes occurring from the LD 1a and being reflected internally of the LD unit 1, etc. The undesired light beam is represented as flare beams FL 1, FL 2 in FIG. 2. These flare beams appear on both sides of the traveling direction of the chief ray L. FIG. 4 shows a magnified view of a part of the optical path of these flare beams. It is often the case that the energy of the first flare beam FL 1 and the energy of the second flare beam FL 2 are smaller than that of the chief ray L. However, if they travel at the same height as the chief ray L, the first flare beam FL 1 passes through the gap between one end of the mirror 4 and the cover 8a of the polygon motor 8 and the second flare beam FL 2 travels on the side of the chief ray L, which side is outer with respect to the other end of the mirror 4 as shown in FIG. 2 and FIG. 4. Both flare beams pass through the Fxcex8 lens 6 shown in FIG. 2 and become flare beams FL 1a, FL 2a that reach the photoconductor drum 7. The flare beams FL 1a, FL 2a always expose themselves to the photoconductor drum 7 as a certain image height and these flare beams form on finally obtained image black lines or black bands in the sub-scanning direction, therefore the image quality is significantly degraded. These flare beams may also be generated at both the upper side and the under side of the chief ray; however, such flare beams are blocked by an optical housing or a cover member that accommodates most of the optical devices and therefore they do not reach the photoconductor drum 7.
Accordingly, it is necessary to prevent the first flare beam and the second flare beam from reaching the photoconductor drum by blocking the relevant flare beams. However, when the first flare beam FL 1 and the second flare beam FL 2 are traveling on the same scanning plane as the chief ray L and the parts FL1a, FL 2a of the first flare beam FL 1 and the second flare beam FL 2 enter the scan region of the chief ray L as shown in FIG. 2, it is difficult to separate the flare beams from the chief ray L. Accordingly, it is necessary to block the first flare beam FL 1 and the second flare beam FL 2 in the optical path between the LD unit 1 and the polygon mirror 5.
In particular, for blocking the first flare beam FL 1, it is necessary to arrange a blocking member between the polygon motor 8 and the mirror 4 avoiding obstructing the traveling of the chief ray L and this is spatially difficult. Further, the polygon motor 8 is removably mounted to the housing of the writing device 10 by screws threaded in the direction perpendicular to the plane of the figure so that the polygon motor can be replaced in case of a failure, etc. However, under a certain design condition, the blocking member may obstruct the removal of the polygon motor and thus may impede a smooth removal of the polygon motor. Therefore, it is extremely difficult to completely block the first flare beam FL 1 by providing a blocking member between the motor 8 and the mirror 4 leaving no space between them, while avoiding obstructing the removal of the motor 8 and blocking the chief ray L.
The present invention is proposed in view of the mentioned problems, and therefore, it is a general object of the present invention to provide an image forming apparatus provided with a writing device with a simple configuration, by means of which writing device the flare beams emitted from the LD unit can be effectively blocked.
According to a first aspect of the present invention, there is provided a device for forming an image on a photoconductive image carrier in an image forming apparatus. The device according to the present invention comprises a light source for generating a light beam, an aperture for shaping the light beam suitable for proper image formation, which light beam is generated by the light source, a deflector assembly for deflecting the light beam shaped by the aperture, an optical scanning imaging part for focusing the light beam deflected by the deflector assembly onto the photoconductive image carrier as an optical spot and forming an image thereon, and a blocking member provided on an optical path between the aperture and the deflector assembly for blocking a light beam that does not contribute to the proper image formation.
In the device according to the present invention, the deflector assembly may comprise a polygon mirror, a polygon motor that rotationally drives the polygon mirror, and a polygon cover that accommodates the polygon mirror and the polygon motor therein, and the blocking member may be integrally formed on the polygon cover.
Therefore, according to the first aspect of the present invention, by providing the blocking member on the optical path between the aperture and the deflector assembly, it is possible to effectively and efficiently block the undesired light beam that dose not contribute to the proper image formation and therefore it is possible to improve the image quality of the finally obtained image.
According to a second aspect of the present invention, there is provided another device for forming an image on a photoconductive image carrier in an image forming apparatus. The device according to the present invention comprises a light source for generating a light beam, a deflector assembly for deflecting the light beam generated by the light source, an optical scanning imaging part for focusing the light beam deflected by the deflector onto the photoconductive image carrier as an optical spot and forming an image thereon, and a blocking member integrally formed on the deflector assembly for blocking a light beam that does not contribute to the proper image formation.
In the device according to the present invention, the deflector assembly may comprise a polygon mirror, a polygon motor that rotationally drives the polygon mirror, and a polygon cover that accommodates the polygon mirror and the polygon motor therein, and the blocking member may be integrally formed on the polygon cover.
In the device according to the present invention, the blocking member may be formed with an opening that allows only a light beam that contributes to the proper image formation to pass therethrough, and the opening may function as an aperture for shaping the light beam suitable for the proper image formation, which light beam is generated by the light source.
Therefore, according to the present invention, an independent aperture may be dispensed with, which aperture is present in the related art, and a fewer number of components may be necessary for the device for forming an image on the photoconductive image carrier according to the present invention.
In the device according to the present invention, the opening formed in the blocking member may have a dimension that can be determined correspondingly to a rotational velocity of the polygon motor.
In the device according to the present invention, the blocking member may be provided on an optical path between the light source and the deflector assembly.
Therefore, according to the second aspect of the present invention, by integrally forming the blocking member on the polygon cover that configures a part of the deflector assembly, it is possible to readily remove the deflector assembly for replacement, etc. without the blocking member obstructing the removal of the deflector assembly.
When the first aspect and the second aspect of the present invention are considered together, it is possible to provide a device for forming an image on a photoconductive image carrier in an image forming apparatus, which device can effectively and efficiently block the undesired light beam that does not contribute to the proper image formation and, at the same time, the deflector assembly that configures a part of the device according to the present invention can be readily removed from the device when necessary without being obstructed by the blocking member.
As a third aspect of the present invention, there is provided an image forming apparatus that comprises a document feeder that feeds document sheets one by one from a pile of document sheets onto a contact glass and sets the document sheet thereon, an image reading unit that optically scans the document sheet and forms an image corresponding to the document sheet on a sensor, an optical scan unit that forms a latent image on a photoconductive image carrier based on a digital image signal provided from the sensor via an analog-to-digital converter, a transfer unit that transfers an image developed from the latent image formed on the photoconductive image carrier onto a transfer paper, and a feeder unit that feeds the transfer paper to the transfer unit. The optical scan unit comprises the above-mentioned device for forming an image on the photoconductive image carrier according to the first and the second aspects of the present invention.
By configuring the image forming apparatus as mentioned above, it is possible to efficiently and effectively block undesired light beams that impede image formation by way of an extremely simple configuration, and thus the image quality of the finally obtained image can be significantly improved.